1. Field Of The Invention:
The present invention relates to a process for producing a magnetic iron oxide powder having high coercive force which is mainly used for a magnetic recording medium.
2. Description Of The Prior Art:
A magnetic recording medium is usually prepared by coating a magnetic powder composition of a mixture of a magnetic powder and a resin binder on a substrate such as a resin film.
Recently, it has been required to increase the coercive force depending upon the increase of uses of such magnetic recording media. Such high coercive force is required for the high density recording for a video-tape, a computer taper and also cassette tape recording at slow speed. In the principle of the magnetic recording, an attenuation of output in the short-wave region is lower depending upon higher coercive force of a magnetic recording medium. Therefore, higher coercive force is suitable for the high density recording. Various manners for increasing the coercive force of the magnetic powder used for magnetic recording media have been proposed.
One of the inventors disclosed the magnetic powder having high coercive force which imparts excellent characteristics when it is used for magnetic recording media, in International Applied Magnetic Society held in 1978. In accordance with the process for producing such magnetic powder, the conventional magnetic iron oxide powder (.gamma.-Fe.sub.2 O.sub.3 etc.) is dispersed into an aqueous solution of a cobalt ion and then, a strong alkaline aqueous solution is added to the suspension to give pH of higher than 12, and the suspension is heated at about 100.degree. C. for several hours and then, the powder is washed with water and filtered and dried. The resulting magnetic iron oxide powder had a coercive force of 550 to 570 Oe and a stable coercive force in variation of temperature. Thus, when the magnetic iron oxide powder is used for magnetic recording media, it is suitable for the high density recording and also it imparts excellent printing effect and excellent erasing property and also excellent durability in the repeated uses and excellent storage stability.
The inventors have further studied improvements of preparation of such magnetic iron oxide powder and have obtained the magnetic iron oxide powder having higher coercive force than that of the conventional magnetic powder.